The use of erbium-doped fiber amplifier technology is increasing within optical fiber communication systems in a wide range of applications in which weak optical signals require amplification. These applications include, but are not limited to, preamplifiers, postamplifiers, and in-line repeaters in optical fiber communication systems.
Also within current optical fiber technology, there is a growing requirement to increase the capacity of the existing communication systems. According to current technology, an increase in capacity can be achieved by increasing the bit rate and/or by adding wavelength division multiplexed (WDM) channels. As a result of the need for more capacity, the use of WDM channels and further, an increased number of such channels are becoming increasingly popular.
FIG. 1 illustrates a typical unidirectional optical fiber communication system in which first and second optical fibers 102,104 couple a wavelength division multiplexer 106 at a first location to a wavelength division demultiplexer 108 at a second location which is remote from the first location. The multiplexer 106 is used to wavelength division multiplex a series of channels (.lambda.1-.lambda.N) and the demultiplexer 108 is used to subsequently demultiplex the WDM channels. As depicted within FIG. 1, coupled between the multiplexer 106 and the first fiber 102 is an EDFA postamplifier 110 and coupled between the second fiber 104 and the demultiplexer 108 is an EDFA preamplifier 112. Further, as depicted within FIG. 1, coupled between the fibers 102,104 is an in-line repeater which comprises an EDFA optical amplifier 114. One skilled in the art would understand that further line-repeaters could also be utilized in such an implementation. This setup is a well understood unidirectional optical fiber communication system.
One major problem in such an implementation as disclosed in FIG. 1 is the non-uniform wavelength dependent gain profile of the EDFA amplifier 114 within the in-line repeater and further within any other EDFA optical fiber amplifiers that may be included between the multiplexer 106 and the demultiplexer 108 such as the post/preamplifiers 110,112. These problems, inherent to the currently utilized EDFA optical fiber amplifiers, result in each channel within a particular WDM system having a different optical gain and a different resulting Optical Signal to Noise Ratio (OSNR). Hence, some channels could have a relatively low OSNR and low received power which, in turn, could result in an excessively high bit error rate.
Considerable efforts are being expended in order to equalize the received powers and OSNRs of the individual WDM channels at the demultiplexer 108 and therefore ensure that all channels have corresponding OSNRs that are above a predetermined allowable threshold level. One technique to equalize the received powers between the channels (.lambda.1-.lambda.N) is to add Variable Optical Attenuators (VOAs) for each channel directly after the demultiplexer 108, so that, within a certain range, the received powers can be adjusted to a common value. Although effective in reducing the difference in received powers, the implementation of these VOAs does not reduce the differences between OSNRs of the individual channels (.lambda.1-.lambda.N).
A technique that is utilized to reduce the difference in received powers and OSNRs between the WDM channels at the demultiplexer 108 is disclosed in U.S. Pat. No. 5,225,922 entitled "Optical Transmission System Equalizer" by Chraplyvy et al, issued on Jul. 6, 1993 and assigned to AT&T Bell Laboratories of Murray Hill, NJ. With this technique, a controller detects the power of the optical signals of each individual channel at each amplifier with use of a series of power detectors and subsequently adjusts the transmission power corresponding to each of the channels at the multiplexer 106 with use of a series of transmission power adjusters. The controller, input with the detected powers, operates to adjust the transmission power for each channel in order to compensate for the non-uniform gain problems caused by the optical fiber amplifiers. Hence, any channels with a low OSNR will have their corresponding transmission power increased while any channels with a high OSNR will have their transmission power reduced. Eventually, this feedback technique will equalize the power corresponding to the received optical signals on all the channels, ensuring that all channels have satisfactory OSNRs and also limiting unnecessary transmission power.
There are a number of key problems with this technique for equalizing the OSNRs corresponding to the individual WDM channels. For one, this feedback technique typically requires numerous iterations, and therefore a considerable amount of time, to complete. This is especially true as the number of channels increase. Secondly, this technique must allow for the transmission power for the individual WDM channels to be adjustable over a large dynamic range. As the dynamic range increases, the complexity and cost of the transmission power adjusters required within the multiplexer 106 also increase.
It can be seen that the unidirectional system of FIG. 1 can be expanded to a typical bidirectional optical fiber communication system as depicted in FIG. 2. This system comprises first and second optical fibers 202,204 coupled between first and second WDM couplers 206,208, each coupler operating as a red and blue band splitter. Further coupled to the first WDM coupler 206 is a blue band signal multiplexer 210 and a red band signal demultiplexer 212, while further coupled to the second WDM coupler 208 is a red band signal multiplexer 214 and a blue band signal demultiplexer 216. The multiplexers 210,214 are used, similar to that for the multiplexer 106 within FIG. 1, to wavelength division multiplex a series of respective channels (.lambda.b1-.lambda.N, .lambda.r1-.lambda.rN) and the demultiplexers 212,216 are used to subsequently demultiplex the channels.
As depicted within FIG. 2, coupled between the first WDM coupler 206 and the first fiber 202 is a blue post/red pre amplifier 218 and coupled between the second fiber 204 and the second WDM coupler 208 is a blue pre/red post amplifier 220. Further, as depicted in FIG. 2, coupled between the fibers 202,204 is a bidirectional in-line repeater 222. It can be seen from FIG. 2 that there is a blue and red transmission path which respectively traverse blue multiplexer 210, WDM coupler 206, blue postamplifier 218, fiber 202, repeater 222, fiber 204, blue preamplifier 220, WDM coupler 208, and blue demultiplexer 216; and traverse red mulitplexer 214, WDM coupler 208, red postamplifier 220, fiber 204, repeater 222, fiber 202, red preamplifier 218, WDM coupler 206, and red demultiplexer 212. One skilled in the art would understand that the key differentiating feature between the red and blue paths is the transmission wavelengths of the corresponding WDM channels, those being in one sample case between 1528 to 1542 nm for the blue path and 1547 to 1561 nm for the red path.
One skilled in the art would understand that the bidirectional repeater 222 of FIG. 2 has similar problems as discussed herein above with respect to the unidirectional repeater 114, hence requiring an equalization technique to be implemented in the bidirectional system. The complexity of such an equalization technique in a bidirectional WDM system increases compared with that in a unidirectional WDM system.
Hence, an improvement in both unidirectional and bidirectional optical fiber communication systems is required that equalizes the OSNRs of the WDM channels in a more efficient manner. Preferably this improvement would reduce the number of iterations required and the dynamic range of the transmission power adjusters. As well, this improvement would preferably not require a significant redesign of the amplifier system, but possibly could take advantage of advancements in two-stage optical fiber amplifier technology to allow for a reduced implementation cost.